1. Field of the Invention
The present invention relates to the field of personal computers and, more particularly, to a real-time standby voltage adjustment circuit for controlling the power saving modes of personal computers.
2. Description of Related Art
Conventionally, a power saving feature is available in the motherboard of a personal computer (PC) for saving power when the PC is not actively used. A plurality of power saving modes, such as S0, S1, S2, S3, S4, and S5, are defined in the power saving scheme of the PC99 specification. Among these power saving modes, S0 represents a power on mode. S1 and S2 define different standby modes. S3 represents a suspend to RAM (STR) mode in which data required to reactivate the PC is stored in the RAM of the PC. S4 represents a suspend to disk (STD) mode in which data required to reactivate the PC is stored in the hard disk of the PC. S5 represents a power off mode.
The achieve the power saving function, the design of the motherboard circuit and the basic input and output system (BIOS) have to be involved. FIG. 3 shows a block diagram of a conventional power saving circuit implemented in a motherboard. As shown, this circuit comprises a voltage adjustment circuit 31, a first switch circuit 32, a second switch circuit 33, and a switch control circuit 34. The voltage adjustment circuit 31 is provided to convert a 5V standby voltage of a power supply (not shown) into a 3.3V standby voltage for being applied to the input of the first switch circuit 32. The input of the second switch circuit 33 is connected to a 3.3V working voltage of the power supply. The switch control circuit 34 generates two complementary switch control signals for the first switch circuit 32 and the second switch circuit 33, respectively, based on the power saving control signal provided by the PC system. As such, when the PC is operating in normal condition, the first switch circuit 32 is off and the second switch circuit 33 is on, such that the 3.3V working voltage of the power supply is output. Conversely, if the PC enters a power saving mode, the first switch circuit 32 is on and the second switch circuit 33 is off, such that the 3.3V standby voltage obtained from the voltage adjustment circuit 31 is output. This 3.3V standby voltage thus supplies a minimum power required for the PC to operate in a power saving mode.
Based on progress of the computer technology and the demand for saving power, the newly manufactured motherboards have to meet the requirement of the PC99 specification that involves the functions of the S0-S5 power saving modes. However, many motherboards fail to pass the PC99 certification test and a message of "The system does not support S4 RTC wake up function, the follow test can't be run" is given. Such a test failure may result from the standby voltage being processed in two stages, that is, sequentially processed by the voltage adjustment circuit 31 and the first switch circuit 32. Therefore, the output voltage is not stable and the timing of the output voltage may not be correct. Accordingly, there is a strong desire to have a novel circuit for the motherboard to support the S4 power saving mode and pass the certification test without modifying the BIOS.